1. Field of Invention
The present invention relates to series-resonant ballast means for gas discharge lamps, and more particularly to means for keeping the ballast substantially in resonance both before and after lamp ignition.
2. Description of Prior Art
A series-resonant ballast refers to an arrangement wherein a gas discharge lamp is connected in parallel with the capacitor of a resonant inductor-capacitor series-combination--where this series-combination is connected directly across an AC supply voltage, and where the natural resonance frequency of the series-combination is substantially the same as the fundamental frequency of this supply voltage.
Series-resonant ballasting principally applies to situations where the frequency of the supply voltage is relatively high (on the order of 20 to 40 kHz), and is described in several prior art references--such as in the following U.S. Patents: No. 3,710,177 to Ward, No. 4,207,497 to Capewell et al, No. 4,207,498 to Spira et al, No. 4,346,332 to Walden, and No. 4,370,600 to Zansky.
Series-resonant ballasting means is generally considered to provide for a good power factor in terms of the power drawn from the power supply by the ballast. After all, a truly resonant circuit draws power with unity power factor.
However, due to the particular nature of the gas discharge lamp, it has not been possible in a series-resonant ballast to maintain a good power factor (i.e., substantial resonance) both before and after lamp ignition. That is: if the ballast circuit is arranged to be in resonance before lamp ignition--which is desirable in order to generate an adequately high lamp starting voltage--it will become non-resonant after the lamp has ignited (it will, in fact, become inductive) and will therefore draw power with a poor power factor; and, if the ballast circuit is arranged to be in resonance during normal lamp operation--which is desirable from a power-factor viewpoint--it will be non-resonant prior to lamp ignition (it will then be capacitive) and will therefore not provide the desired high lamp starting voltage.
Of course, in some situations the problem might be resolved by providing for the frequency of the supply voltage to decrease after lamp ignition; but that is not easy to accomplish in a cost-effective manner--especially not when several ballast circuits are operated from the same supply voltage.